The present invention relates to reduction of local heating in an optical device, and particularly reduction of local heating by using a thermally conducting electrode.
Devices such as lasers, optical modulators and photo detectors experience heating due to the nature of the physics involved in their operation.
An example in which local heating is problematic occurs in optical modulator devices. An optical modulator such as an electro-absorption modulator includes a waveguide through which light is directed. Light is normally allowed to pass through the device. An electrode on top of the waveguide is used in conjunction with another electrode at the bottom of the device to introduce an electric field in the waveguide. This electric field changes the semiconductor properties so that light passing through the waveguide is absorbed. This absorption of the light results in heating of the device. Overheating of the device will cause breakdown of one or more components and result in failure of the device.
The electrode itself contributes to dissipation of heat in the device. In particular, in a region of the waveguide where the electrode makes contact with the waveguide, the electrode acts as a thermal conductor and creates a thermal path which can carry heat away from that portion of the waveguide thereby reducing the possibility of overheating. However, the electrode does not necessarily extend the entire length of the waveguide as this can have undesirable effects such as increasing capacitance, thereby decreasing the speed at which the modulator reacts. For example, the electrode often does not extend to the point of optical entry of light into the device. In areas not covered by the electrode there is reduced thermal conduction. As a result, heat is not dissipated as efficiently and local overheating of the waveguide in such an area can result in catastrophic failure of the device.
Accordingly, such devices must be used within certain operating limits or constraints such as limiting the amount of optical power that can be delivered in the device, or limiting the amount of electrical bias which can be applied. These constraints can limit the range of applications to which the device can be put, particularly in evolving high power optical networks.
Another problem which exists in such devices is the termination of the electrode. For example, in an InGaAs electro-absorption modulator the electrode can be formed of several layers of metal, including a gold layer. Abrupt termination of the electrode can permit gold to diffuse into the device, rendering it inoperative. Therefore, electrodes in such devices are typically raised at their termination from the semiconductor layer. Although techniques for terminating the electrode in a raised fashion away from the contact layer of the device prevent such diffusion, the requirement to do so is undesirable and increases the complexity of the manufacturing process. A further disadvantage of this termination technique is that the raised portion of the electrode does not serve to conduct heat away from the device.
Overheating also exists in lasers such as the 980 nm pump laser. The 980 nm pump laser is a semiconductor laser having a waveguide which terminates in an exposed cross-section, known as a facet. In particular, overheating is a problem in the vicinity of the facet because the facet forms an interface with the air. A technique to mitigate the interface problem is to specially alter a portion of the waveguide near the facet so that it is not energized or xe2x80x9cpumpedxe2x80x9d and does not act as a laser. The modified portion of the waveguide, for example the first 100 microns of the waveguide from the facet, is known as a window. However, even with this approach, local overheating may still be a problem. Similar to the example of the optical modulator, if an electrode stripe terminates before the facet region, then that region will not does not have an efficient thermal path to conduct heat away.
Accordingly, it is an object of the present invention to provide a device and method to overcome or mitigate at least one of the disadvantages of previous methods and devices for reducing localized heating in optical devices.
According to an aspect of the present invention, a device for reducing localized heating in an optical waveguide includes an electrode for applying an electrical bias across the optical waveguide and a contact layer for providing an interface or common boundary between the electrode and the optical waveguide. The contact layer has an electrically and thermally conductive first region, and a thermally and non-electrically conductive second region for dissipating heat in a region of localized heating.
According to another aspect of the present invention, the thermally and non-electrically conductive second region, as discussed above, includes an isolation dielectric, such as SiN. The electrode terminates in a terminal end which is in abutting contact with the second region.
According to a further aspect of the present invention, a device for reducing localized heating in an optical waveguide includes a contact layer on the optical waveguide. The contact layer which extends longitudinally along a portion of the waveguide has an electrically and thermally conductive first region, and a thermally and non-electrically conductive second region for dissipating heat in a region of localized heating. A thermally conducting electrode extends longitudinally along the contact layer.
According to a still further aspect of the present invention, there is provided a method for reducing localized heating in a device through which light passes. The device includes a waveguide on a semiconductor substrate. The method includes providing an electrode adjacent the waveguide for applying an electric field. A thermally conducting and electrically conducting region is provided between the waveguide and the electrode in a region of desired electrical contact between the electrode and the waveguide while a thermally conducting and electrically insulating region is provided between the waveguide and the electrode outside the region of desired electrical contact between the electrode and the waveguide.
According to a still further aspect of the present invention, a method of manufacturing a device for controlling light which passes through the device is provided. The method includes providing an optical waveguide on a semiconductor substrate and adding to the surface of the waveguide a contact layer of electrically conducting and thermally conducting material. The contact layer is then masked to define a first region of the contact layer and leaving exposed a second region of the contact layer. The electrically conducting material is then etched away in the exposed region of the contact layer. A blanket deposit of an electrically insulating and thermally conducting material is added to the first region and the etched away second region of the contact layer. Then the second region is masked leaving exposed at least a part of the first region of the contact layer. The electrically insulating and thermally conducting material is etched away in the exposed part of the first region of the contact layer. Metal is then deposited onto the contact layer to form an electrode.
Reduction of local heating according to the present invention permits greater optical power to be delivered to the device. Another advantage is that for a given level of optical input, a higher reverse bias voltage can be used to modulate the light signal thereby permitting increased speed of the device. Accordingly, the device can be used in a greater variety of situations or for a greater variety of purposes as the previously existing operating constraints have been lessened.